Southerners said the proposal to limit farmers to $250,000 a year in subsidies, down 30 percent from the current limit, was an attack on their rice and cotton growers. The crops have the highest subsidy rates but also cost more to grow.
"This amendment is about the family farm," said sponsor Charles Grassley. The Iowa Republican said big farmers used their subsidies to outbid family farmers for land and equipment.
The Senate was scheduled to vote on the proposal for a lower limit on Thursday as part of work on a government-wide bill to cut some $35 billion in spending. If it approves the bill, a compromise would have to be written with a somewhat different House bill.
"You can't just change the rules from one year to the next" and have a vibrant farm sector, said Mississippi Republican Thad Cochran. He predicted "a serious and adverse effect on farmers in Southern states in particular" if the current $360,000 limit was changed.
Subsidy caps are one of the most divisive issues in modern agriculture.
They pit small growers against big-scale operators and the Northern states which grow lower-cost corn, wheat and soybeans against rice and cotton in the South.
Ten percent of US growers collect 72 percent of farm subsidies.
President Bush supports a $250,000 limit. Budget hawks, small-farm activists and farmland conservation groups say a lower limit would save $1 billion over 10 years, aid small farmers and funnel more money into land stewardship.
"This is the easiest decision ever offered," said proponent Byron Dorgan, North Dakota Democrat. He said the $250,000 limit would "shut off the spigot" of tax dollars to big farmers.
Southern senators argued no change should be considered until Congress updates the entire farm subsidy law in 2007. It was unfair to talk about smaller subsidies, they said, in the face of hurricane damage and rising fuel and fertiliser prices.
"This amendment would lump cotton and rice into the same category as crops that require half those inputs," said Arkansas Democrat Blanche Lincoln.
Senate Agriculture Committee chairman Saxby Chambliss, Georgia Republican, said Iowa farmers got 10 percent of all farm subsidies last year.
Farm groups generally opposed the $250,000 limit. Senators approved a similar amendment three years ago by a margin of 66 to 31, but it never took effect. More recent attempts have foundered.